This is a loaded topic...but here's my situation (Im assuming its not the same as yours):
I've been selling info on eBay for a few years. I no longer leave feedback right after the person pays mainly due to dishonest people that purchase for the purpose of leaving negative feedback (unethical competitors selling a similar product crate accounts that are used as feedback assisns). That sounds funny right? But 1 neg feedback costs me a few hundred buck minimum - they could have written I LOVE BUNNIES next to the red mark. It doesnt matter. Red = bad person. So far, I have only gotten negs from competitors & 1 nut over a bridesmaid dress. So thats my personal reason.
I doubt that is your situation - but I've been burned enough times that I no longer automatically leave feedback. I mention it at checkout and don't bother people past that about it.
Ebay has become a slimy place for buyers and sellers. It used to be nice and a good spot to buy a bargain. Now I have to weed to find good stuff to buy and Im paranoid of thieves lurking in there.
I bought a bridesmaid gown (new - exactly what I needed) and the girl never shipped the dress. I paid for insurance and tracking #. She didnt have either. She went nuts when I requested that she file the insurance claim so I could get my money back and buy another dress. She topped it off with leaving me negative feedback.
Remember - not everyone sucks - just about everyone does.
Ebay has been a pet peeve here too. I feel for ya.